function fn() {
  this.a = 0;
  this.b = function () {
    console.log("-1-")
    alert(this.a)
  }
}
fn.prototype = {
  b: function () {
    console.log("-2-")
    this.a = 20;
    alert(this.a)
  },
  c: function () {
    this.a = 30;
    alert(this.a)
  }
}
var myFn = new fn()
console.log(myFn)
// myFn.b()
// myFn.c()

// 实例 myFn  类 fn
// fn.prototype === myFn.__proto__

